Catalytic hydrogen peroxide decomposer in water-cooled reactors

ABSTRACT

A metal cooling tube of a water-cooled nuclear reactor, having an inner surface thereof exposed to an aqueous cooling medium containing hydrogen peroxide. The cooling tube has its inner surface coated with matter selected from the group consisting of the element manganese, molybdenum, zinc, copper, cadmium for absorbing such hydrogen peroxide and then affecting decomposition of the hydrogen peroxide in the aqueous medium. In preferred embodiment such coating is manganese and oxides thereof. A method for lowering the electrochemical corrosion potential of a metal allow cooling tube exposed to an aqueous medium in a water-cooled nuclear reactor is also disclosed. Such method comprises the step of coating an inner surface of such tube with matter selected from the group of elements comprising manganese, molybdenum, zinc, copper, cadmium, so as to permit absorption and hydrogen peroxide in such aqueous medium and effect decomposition of hydrogen peroxide in such aqueous medium.

This application is a division of application Ser. No. 09/259,645 andU.S. Pat. No. 6,259,758, filed Feb. 26, 1999, which is herebyincorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

The present invention is directed to hydrogen peroxide decomposer foruse in water-cooled nuclear reactors, including boiling water reactorsand pressurized water reactors, for the mitigation of corrosionphenomena in such systems.

Steel pressure vessels and piping exposed to high temperature water areprone to corrosion due to oxidation of the various metals therein byoxidizing agents, particularly oxygen, present in the high temperaturewater. Corrosion of such vessels and piping can lead to a variety ofproblems, including stress corrosion cracking, crevice corrosion anderosion corrosion, leading to leakage and/or bursting of such vesselsand piping.

In nuclear reactors, significant amounts of heat energy is generated byreactor processes occurring in the reactor core. A liquid coolant,typically water, is used to remove heat from the reactor core andfacilitate its conversion to a useable form. A reactor vessel isprovided to contain the reactor coolant around the reactor core toeffect such heat removal. Further, piping is provided to facilitatetransport of the coolant to steam generators or turbines, where heatenergy is ultimately converted to electricity. The materials used in theconstruction of nuclear reactor vessels and piping are elected for theirability to withstand rigorous loading, environmental and radiationconditions. Such materials include carbon steel, low alloy steel,stainless steel and nickel-based, cobalt-based and zirconium-basedalloys.

Despite careful material selection, corrosion and, particularly,intergranular stress corrosion cracking (or, simply, stress corrosioncracking (SCC)), is a problem in steel pressure vessels and piping usedin nuclear reactors. SCC, as used herein, refers to cracking propagatedby static or dynamic tensile stressing in combination with corrosion atthe crack tip. Unfortunately, the nuclear reactor environment isconducive to both tensile stressing and corrosion.

Nuclear reactor pressure vessels and piping are subject to a variety ofstresses. Some are attributable to the high operating pressure requiredto maintain high temperature water in a liquid state. Stresses alsoarise due to differences in thermal expansion of the materials ofconstruction. Other sources include residual stresses from welding, coldworking, and other metal treatments.

Nuclear reactors are also susceptible to SCC because of the waterchemistry environment of its process systems, which is favourablydisposed to corrosion. In this respect, the presence of oxidizingagents, such as oxygen, hydrogen peroxide, and various short-livedradicals, which arise from the radiolytic decomposition of hightemperature water in boiling water reactors, contribute to SCC.

Hydrogen peroxide is particularly unstable as it has the ability to actas both an oxidizing agent and a reducing agent. Hydrogen peroxide canact as an oxidizing agent, leading to the formation of water accordingto the following reaction:

H₂O₂+2H⁺+2e⁻→2H₂O

As a reducing agent, hydrogen peroxide is oxidized to oxygen accordingto the following reaction:

H₂O₂→O₂+2H⁺+2e⁻

Because of its ability to act as both an oxidizing agent and a reducingagent, hydrogen peroxide is highly unstable and will spontaneouslydecompose into water and oxygen according to the following reaction:

2H₂O₂→2H₂O+O₂

This will happen if aqueous hydrogen peroxide contacts a metallicsurface whose electrode potential lies within this region ofinstability, which is typically the case in the BWR environment.

Stress corrosion cracking is of great concern in boiling water reactors(BWR's) which utilize light water as a means of cooling nuclear reactorcores and extracting heat energy produced by such reactor cores. Stresscorrosion cracking causes leakage or bursting of such vessels or pipingresulting in the loss of coolant in the reactor core. This compromisesthe reactor process control, which could have dire consequences.

To mitigate stress corrosion cracking phenomenon in BWR's, it isdesirable to reduce the electrochemical corrosion of metal componentsthat are exposed to aqueous fluids. ECP is a measure of thethermodynamic tendency for corrosion to occur, and is a fundamentalparameter in determining rates of stress corrosion cracking. ECP hasbeen clearly shown to be a primary variable in controlling thesusceptibility of metal components to stress corrosion cracking in BWRs.FIG. 1 shows the observed and predicted crack growth rate as a functionof ECP for furnace sensitized Type 304 stainless steel at 27.5 MPa in288° C. water over the range of solution conductivities from 0.1 to 0.3μS/cm.

For type 304 stainless steel (containing 18-20% Cr, 8-10.5% Ni, and 2%Mn), it is known that if the ECP of such steel exposed to hightemperature water at about 288° C. can be reduced to values below −230mV (Standard Hydrogen Electrode—SHE) (hereinafter the “criticalcorrosion potential”), the stress corrosion cracking problem of suchsteel can be greatly reduced. The same generally applies for other typesof steels.

A well-known method to reduce the ECP to less than −230mV_(SHE) andthereby mitigate SCC of steel pressure vessels and piping in nuclearreactors, is to inject hydrogen gas to the recirculating reactorfeedwater. The injected hydrogen gas reduces oxidizing species in thewater, such as dissolved oxygen. This has the very desirable benefit ofreducing the corrosion potential of the steel vessel or piping carryingsuch high temperature water.

As illustrated in FIG. 2, ECP of 304SS in 288° C. water increases morerapidly with continued addition of hydrogen peroxide when compared tothe ECP values measured at the same levels of oxygen concentration.Further, even with the use of hydrogen gas injection, SCC in BWRscontinues to occur at unacceptable rates when hydrogen peroxide ispresent. This is illustrated in FIG. 3, where stress corrosion crackingis shown to occur in BWRs, even with the addition of hydrogen gas, when20-30 ppb of hydrogen peroxide is present. This information suggeststhat the presence of hydrogen peroxide in reactor systems is asignificant contributor to stress corrosion cracking of metalcomponents. Moreover, the present practice of injecting hydrogen gasinto the process liquid does not appear to completely assist in thedecomposition of hydrogen peroxide and therefore does not bring aboutthe concomitant reduction in ECP that is expected.

SUMMARY OF INVENTION

In one broad aspect, the present invention provides a corrosionresistant alloy having a surface exposed to aqueous liquid consisting ofoxidizing species, including hydrogen peroxide, that increase the ECP ofthe alloy. The surface of the alloy is coated with a coating comprisedof Mn, Mo, Zn, Cu, Cd, oxides thereof, or chemical compounds thereof.These metals and their compounds assist in causing the decomposition ofhydrogen peroxide, thereby reducing the ECP of the alloy. These metalsand their compounds can be present as a pre-existing coating on thealloy, or may be deposited in-situ into the aqueous liquid forsubsequent deposition on the surface of the alloy after injection.

According to another broad aspect of the present invention there isprovided a corrosion resistant alloy cooling tube in a water-coolednuclear reactor having a surface exposed to an aqueous cooling mediumcontaining hydrogen peroxide, the surface being coated with a coatingcomprising matter selected from the group consisting of manganese,molybdenum, zinc, copper, cadmium, oxides thereof, chemical compoundsthereof and mixtures thereof, for causing decomposition of the hydrogenperoxide.

According to another aspect of the present invention there is provided awater-cooled nuclear reactor comprising metal piping, such metal pipinghaving a surface exposed to an aqueous liquid containing hydrogenperoxide, the surface being coated with a coating comprising matterselected from the group consisting of manganese, molybdenum, zinc,copper, cadmium, oxides thereof, chemical compounds thereof and mixturesthereof, for causing decomposition of the hydrogen peroxide.

According to another aspect of the present invention there is provided amethod for lowering the electrochemical corrosion potential of a metalalloy, for use in a cooling tube in a water-cooled nuclear reactor,having a surface exposed to an aqueous liquid containing hydrogenperoxide, comprising the step of coating the surface with matterselected from the group consisting manganese, molybdenum, zinc, copper,cadmium, oxides thereof, chemical compounds thereof and mixturesthereof, for causing decomposition of the hydrogen peroxide.

In a further aspect of the present invention, there is provided a methodof lowering the electrochemical corrosion potential of metal alloycooling tubes in a water-cooled nuclear reactor, the tubes havingsurfaces exposed to an aqueous liquid containing hydrogen peroxide,comprising the step of injecting matter into said water, said matterselected from the group consisting of manganese, molybdenum, zinc,copper, cadmium, oxides thereof, chemical compounds thereof and mixturesthereof, for causing decomposition of the hydrogen peroxide.

BRIEF DESCRIPTION OF THE DRAWINGS

The method and apparatus of the invention will now be described withreference to the accompanying drawings, in which:

FIG. 1 is a graph illustrating the observed and theoretical crackpropagation rate as a function of electrochemical corrosion potentialfor sensitized 304 stainless steel in 288° C. water under constant load(25 ksiin);

FIG. 2 is a graph illustrating electrochemical corrosion potential for304 stainless steel in 288° C. water containing various amounts of H₂O₂showing corrosion potential of such 304 stainless steel as a function ofthe O₂ and H₂O₂ concentration in water.

FIG. 3 is a graph illustrating the electrochemical corrosion potentialfor 316 stainless steel in 288° C. water containing various amounts ofH₂O₂ showing corrosion potential of such 316 stainless steel in as afunction of dissolved H₂ concentration in water with varying amounts ofhydrogen peroxide.

FIG. 4 is a graph illustrating the electrochemical corrosion potentialof 304 stainless steel in 288° C. water containing 300 parts per billionhydrogen peroxide with and without in-situ injection of variousmanganese concentrations as a function of immersion time.

FIG. 5 is a graph illustrating the electrochemical corrosion potentialof 304 stainless steel in 288° C. water containing 100 parts per billionhydrogen peroxide with and without in-situ injection of various zincconcentrations as a function of time.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

It has been found that, by coating an alloy from the group consisting ofcarbon steel, alloy steel, stainless steel, nickel-based alloys,zirconium and cobalt-based alloys with a catalytically active material,or otherwise providing catalytic activity at such metal alloy surfaces,the decomposition of the hydrogen peroxide in aqueous process systems ofnuclear reactors is catalysed by the catalytically active material. Suchcatalytic action at the surface of the alloy reduces the ECP of thealloy, thereby mitigating SCC of such alloy. Suitable coatings ofcatalytically active material can be deposited by methods well known inthe art for depositing continuous or substantially continuous coatingson metal substrates, such as plasma spraying, chemical vapourdeposition, physical vapour deposition processes such as sputtering,welding such as metal inert gas welding, electroless plating, andelectrolytic plating.

The catalytically active material can be a metal selected from the groupconsisting of manganese, molybdenum, zinc, copper, cadmium and mixturesthereof. Other suitable materials include oxides of these metals. Evenfurther suitable materials can include chemical compounds containingthese metals, where the metal in such compounds is able to dissociateand make itself available for reacting with oxygen to form an oxide.

Manganese dioxide catalyzes the decomposition of hydrogen peroxideaccording to the following reaction mechanism:

H₂O₂+MnO₂+2H⁺→O₂+Mn²⁺+2H₂O

Mn²⁺+2H₂O₂→Mn(OH)₂+2H⁺

Mn(OH)₂+H₂O₂→MnO₂+2H₂O

2H₂O₂→O₂+2H₂O

It is believed that, by coating the surface of a metal alloy coolingtube of a water-cooled nuclear reactor with manganese, such component isable to maintain a lower ECP. This is because the manganese is believedto be oxidized to catalytically active manganese oxide (MnO₂), whichcatalyses hydrogen perixode decomposition.

Because very small surface concentrations are adequate to provide thenecessary catalytic activity and reduce the corrosion potential of themetal, the processing as well as the physical, metallurgical ormechanical properties of the alloys and components formed therefrom arenot significantly altered. Further, lower amounts of reducing species,such as hydrogen, are necessary to reduce the ECP of the metalcomponents below the critical potential, because of the catalyseddecomposition of hydrogen peroxide.

As an alternative to coating the subject alloy with the catalyticallyactive material, the catalytically active material may be injectedin-situ in the process liquid for effecting decomposition of hydrogenperoxide, thereby reducing the ECP of the alloy. FIG. 4 shows thebenefits of in-situ injection of manganese as Mn(NO₃)₂. 6H₂O foreffecting decomposition of hydrogen peroxide. With each injection, therewas a corresponding reduction in ECP of the alloy believed attributableto the decomposition of hydrogen peroxide. It is believed that theinjected manganese oxidizes and precipitates out as MnO₂ on the alloysurface. Once deposited on the surface, MnO₂ effects the catalyticdecomposition of hydrogen peroxide according to the above-describedreaction mechanism.

The present invention will be described in further detail with referenceto the following non-limitative examples.

EXAMPLE 1

A 304 SS electrode was placed in an autoclave recirculating loop,containing water at 288° C. having 300 ppb hydrogen peroxide. Variousconcentration of dissolved Mn solution were injected directly into theautoclave where the 304 SS electrode was immersed and argon gas wascontinuously purged through this injection solution during the test. TheECP of the 304 SS electrode was measured over the course of 30 daysusing a Cu/Cu₂O/ZrO₂ electrode. The measured ECP was converted to astandard hydrogen electrode (SHE) scale.

FIG. 4. shows the ECP response of 304 SS electrode before, during, andafter three different manganese solution injections to 288° C. watercontaining 300 ppb hydrogen peroxide. It is evident that the addition ofMn to 300 ppb hydrogen peroxide water decreased the ECP of the 304 SSelectrode. Once injections were ceased, the ECP of the 304 SS electroderemained lower than the corrosion potential observed before theinjections were commenced. This indicates the possible deposition ofmanganese oxide on 304 SS oxide, with the concomitant catalyticdecomposition of hydrogen peroxide by the deposited manganese. Thepresence of manganese was, in fact, confirmed by Auger electronspectroscopy, which confirmed a thin oxide layer of 2˜4% by weight onthe 304 SS surface, to a depth of 100˜150 A.

From the above test, the presence of manganese oxide on the metalsurfaces enhances the decomposition of hydrogen peroxide, with aconsequent decrease in ECP of the metal alloy.

EXAMPLE 2

A 304 SS electrode was placed in an autoclave recirculating loop,containing water at 288° C. having 100 ppb hydrogen peroxide. Zinc, aszinc oxide, was injected directly into the autoclave where the 304 SSelectrode was immersed. The ECP of the 304 SS electrose was measuredover the course of 25 days using a Cu/Cu₂O/ZnO₂ electrode. The measuredECP was converted to a standard hydrogen electrode (SHE) scale.

FIG. 5 shows the ECP response of 304 SS electrode before, during andafter aqueous zinc oxide injection to 288° C. water containing 100 ppbhydrogen peroxide. Clearly, once injection of the aqueous Zinc oxidebegan, ECP of the 304 SS became reduced. Once injection was stopped, theECP of the 304 SS electrode remained lower than the corrosion potentialobserved before the injections were commenced. This indicates thepossible deposition of zinc oxide on 304 SS, with the concomitantdecomposition of hydrogen peroxide by the deposited zinc oxide.

The present invention provides a number of important advantages. Inparticular, the present invention provides a metal alloy surface coatedwith a catalytically active material for the decomposition of hydrogenperoxide. By doing so, the ECP of such metal alloys is lowered, therebyreducing corrosion and, notably, mitigating the effects of stresscorrosion cracking. This is particularly beneficially for components ofwater-cooled nuclear reactors, whose high temperature aqueousenvironment is conducive to such corrosion phenomena, and where theoccurrences of such phenomena could lead to loss of coolant andconsequent loss of reactor control.

It will be understood, of course, that modifications can be made in theembodiments of the invention described herein without departing from thescope and purview of the invention. For a complete definition as to thescope of the invention, reference is to be made to the appended claims.

We claim:
 1. A method of lowering the electrochemical corrosionpotential of metal alloy cooling tubes in a water-cooled nuclearreactor, said tubes having a surface exposed to an aqueous liquidcontaining hydrogen peroxide, comprising the step of injecting matterinto said liquid, wherein said matter is selected from the groupconsisting of elemental manganese, elemental copper, elemental cadmium,oxides thereof, and mixtures thereof.
 2. The method as claimed in claim1 wherein said metal alloy is selected from the group consisting ofcarbon steel, alloy steel, zirconium stainless steel, nickel-basedalloys, cobalt-based alloys and mixtures thereof.
 3. The method asclaimed in claim 1 wherein said matter is capable of adsorbing saidhydrogen peroxide.
 4. The method as claimed in claim 3, wherein saidmetal alloy is a zirconium alloy and said matter is selected from thegroup consisting of elemental manganese, oxides thereof, and mixturesthereof.